Most events run each tick (every time the cpu runs through the event sheet). There are some events that inherently only run a single time (on mouse clicked, on collision, etc). The rest of them happen repeatedly. I suspect the sound is getting played repeatedly and the stopping is only stopping a single instance of it. If you have an event that you want to force to play once (something like "If variable > 4, play sound"), you can go to System>Trigger Once While True to make that event only run a single time instead of repeatedly.